Liquid crystal display devices are now being widely used in electronic apparatuses such as a monitor, a projector, a mobile phone, and a PDA (personal digital assistant), because of its characteristics such as slim profile, light weight, and low power consumption. In particular, an active matrix liquid crystal display device is being widely used because good image display without any cross talk between adjacent pixels can be achieved.
The active matrix liquid crystal display device is typically provided with a liquid crystal display panel in which a liquid crystal layer is held via an alignment film between a thin film transistor array substrate (hereinafter, also referred to as a “TFT (Thin Film Transistor) array substrate” and a counter substrate which are disposed at a predetermined interval. The electric field intensity, which is applied between a transparent pixel electrode of each pixel region formed on the TFT array substrate side and a common electrode formed on the counter substrate side, is controlled, and the alignment of the liquid crystal in each pixel region is changed. Thereby, the light transmittance is changed, and the image is displayed.
The counter substrate is typically a color filter substrate (hereinafter, also referred to as a “CF substrate”) provided with a color filter layer of any one of three colors, R (red), G (green), and B (blue), in each pixel region. In the CF substrate, each pixel region is separated by a light-shielding film (hereinafter, also referred to as a “black matrix”). Regarding the light source of the liquid crystal display device, the transmissive liquid crystal display device provides image display by guiding light from the back side, such as light from a backlight that is arranged on the back side of a liquid crystal display panel, into the inside of the panel, and externally outputting the light.
The TFT array substrate comprises source lines and gate lines that are disposed on a main surface of a transparent substrate in a lattice form. A transparent pixel electrode is disposed in a pixel region defined by the source lines and the gate lines. A thin film transistor (hereinafter, also referred to as a “TFT”) is disposed in the vicinity of an intersection between the source lines and the gate lines. The TFT comprises a gate electrode connected to the gate lines, a source electrode connected to the source lines, and a drain electrode connected to the transparent pixel electrode, and further comprises a semiconductor layer made of an amorphous silicone (a-Si). The gate electrode is covered with the gate insulating film (not illustrated). The TFT controls the transparent pixel electrode individually and selectively.
A storage capacitor wiring (hereinafter, also referred to as a “Cs wiring”) is disposed between the gate lines so as to cross the source lines. Multiple Cs wirings are formed in parallel with each other on the TFT array substrate, and the identical Cs wiring is shared with the same line of pixels among multiple pixels disposed in a matrix pattern. The Cs wiring is usually overlapped with the transparent pixel electrode. A storage capacitance for maintaining a drain voltage is obtained by using part of the Cs wiring and transparent pixel electrodes as electrodes and using the interlayer insulating film as a dielectric to produce a capacitor.
In the TFT array substrate having the above configuration, the region including the transparent pixel electrode is used as a display region of the liquid crystal display. However, the source lines, the gate lines, the Cs wiring, and the storage capacitor electrode are formed of a metal material (for example, copper (Cu) and silver (Ag)) for achieving lower resistance. Similarly, an electrode part, such as a gate electrode, of the TFT is formed of a metal material. Accordingly, the region including these wirings serves as a light-shielding region irrespective of the alignment of the liquid crystal and is regarded as a non-display region even in a pixel region, and therefore may not be effectively employed as a display region.
Therefore, in the active matrix liquid crystal display device provided with the TFT array substrate, the area of the light-shielding region formed by each of the above wirings is required to be reduced in order to improve the ratio of the display region to all the display screens, that is, the “aperture ratio” in the pixel region.
However, in the case where the width of each of the wirings is decreased in order to improve the aperture ratio, the wiring resistance may increase. In the case of the Cs wiring, a storage capacitance necessary to prevent the cross talk and flicker of the liquid crystal display device decreases. This tendency is remarkable in the liquid crystal display device having fine pixels in particular, and the aperture ratio of the pixel region is required to be increased while the storage capacitor is secured.
Patent Document 1, for example, proposes a liquid crystal display device having a wiring/electrode on an insulating substrate. The wiring/electrode is formed by covering a laminated structure film with a transparent conductive film. The laminated structure film is formed by covering an aluminium layer or an alloy layer mainly comprising aluminium with a high-melting-point metal layer.
However, the liquid crystal display device having such a configuration essentially comprises a laminated structure film in which the alloy layer is covered with the high-melting-point metal layer. Accordingly, the step of producing a laminated structure film is further needed in addition to the step of producing a conventional TFT array substrate. Since the increase in the number of production steps leads to increases in processing cost, material cost, and the like, the aperture ratio is preferably improved in a simple step. In the case that the aluminium layer or the alloy layer mainly comprising aluminium is formed in the gate electrode part of the TFT, acid or alkali may generate electrolytic corrosion. In addition, the transparent conductive film formed on the laminated structure film is covered with the insulating film. In the case that a transparent conductive film, and then an insulating film are formed, the heat (about 350° C.) added upon forming an insulating film may change the composition of the transparent conductive film.
[Patent Document 1]
Japanese Kokai Publication No. 2001-194676